Algae bloom in the Baltic Sea. Satellite image of a phytoplankton bloom (green swirls) in the nutrient-rich waters around Sweden's Gotland island in the Baltic Sea. Phytoplankton are microscopic marine plants that form the basis of nearly all ocean food chains. Such blooms occur when deep currents bring nutrients up to sunlit surface waters, fueling the growth and reproduction of the phytoplankton. Image obtained by Landsat 7 satellite on 13 July 2005.